1. Field of the Invention
The present invention relates to a method and apparatus for generating a halftone image. The present invention also relates to a computer program product for performing the method for generating a halftone image.
2. Description of Background Art
Digital halftoning is a technique employed in the field of image processing in order to generate a halftone output image from a continuous-tone image. Particularly, digital halftoning is used in printers with binary or limited level output such as inkjet printers and the like for enabling the printing of continuous-tone images such as 8-bit grey-scale images or 24-bit color-scale images. Common methods according to the background art for generating a multilevel halftone image include error diffusion and periodic dithering.
Halftoning based on error diffusion has become one of the most popular techniques for producing a halftone output image from a continuous-tone image. Particularly in the field of computerized images, wherein for example 8-bit grey scale or 24-bit color value images are common, error diffusion halftoning is used widely when halftoned images are to be printed using multilevel output printers. A binary output printer is an example of a multilevel output printer.
A method for generating a multilevel halftone image of the above type is known from EP 0544511 A2. The function that determines the error diffusion threshold level is the difference between a threshold value selected from a set of threshold values and the input pixel value multiplied by a selected enhancement factor. Said selected enhancement factor determines how closely image detail is preserved, and enables, for example, to improve the apparent sharpness of the image. The introduction in the function of a threshold value selected from a set of threshold values contributes to a periodic modulation of the error diffusion threshold level.
With the method disclosed in EP 0544511 A2, it has been noticed that unwanted textures appear in the halftoned image, such as bands in the mid-tones and wavy lines (often called worms) in the highlights and shadows.